Downer is a promising reactor for coal pyrolysis, where the energy used for coal pyrolysis comes from the heat carrier particles. However, fundamental studies are limited regardless of its practical importance. To this end, a multifluid model is established to study the hydrodynamics, heat, mass transfer, and chemical reaction in a downer reactor: The conservation equations of mass, momentum, and energy are closed with proper models for interphase mass, momentum, and heat transfer, including the particle radiation mechanism and with advanced kinetic theory for particulate phase stresses and particle-particle drag coefficients. Species transport equations, the reaction kinetic model, and the water evaporation model are also coupled. Computational fluid dynamics simulations highlight the importance of feeding temperature in coal pyrolysis and indicate that the gas-solid heat transfer and particle radiation are the major heat transfer mechanisms, whereas the direct particle-particle heat transfer is negligible. The simulations prove qualitatively the rationality of the model for coal pyrolysis.